(1) Field of the Invention
The invention relates to a digital filter having a finite impulse response, comprising means for receiving applied digitally encoded signal samples of an input signal, means for storing filter coefficients, means for forming products of received signal samples and stored filter coefficients and means for forming a sum of a plurality of said products for producing an encoded signal sample of an output signal.
(2) Description of the Prior Art
For each output sample of a digital filter of the type having a finite impulse response (FIR-filter), a sum of products of filter coefficients and input samples must be calculated.
When only a small number of clock instants are available for calculating the output sample and in addition little material (hardware) is available, it will be difficult to satisfy high filter requirements.
It is already known from U.S. Pat. No. 3,965,338 to use a bit position shifting arrangement instead of a multiplier in order to reduce the required hardware. It is further known from U.S. Pat. No. 3,988,606 to encode the filter coefficients as a combination of the lowest possible number of powers of the base number 2, as a result of which the number of required clock instants for calculating an output sample is reduced.
Employing the principles of these patent specifications, it is, for example, possible to calculate in 62 clock pulses an output sample of a filter having 40 filter coefficients (40-point filter) by the choice that 18 coefficients are a power of the base number 2 and 22 coefficients are a combination of two powers of the base number 2. In practice, it may be found that this encoding operation of the filter coefficients is too coarse to realize the filter requirements imposed.